I find that calling large multi-national corporations like Yamaha is faster than email. I would call your local Yamaha service branch. There should be a contact number listed on the Yamaha website under your country's support page.

Yamaha tends to keep an excellent stock of most synth parts going all the way back to the original DX7 MkI. There are some things that are no longer available, but that slider button should be an easy find. The first place to check for parts is the original synth manufacturer. And I too have found that phone calls are far more effective than emails.Clyde